In the production of steel products it is desirable to detect manufacturing flaws as soon as possible so that remedial steps can be taken to eliminate the cause of those flaws, and to repair a flawed workpiece or reject it before it is further processed. The process steps in manufacturing steel are well documented and will not be described in detail. Briefly, molten iron is treated with oxygen and other agents are added in varying amounts to produce molten steel which is poured into molds to produce ingots or a continuous caster to produce slabs or billets. Each billet in turn is heated and shaped into a bloom, a large block of steel anywhere from 10" to 16" in thickness and 20' to 30' in length. These blooms are stored and reheated when further shaping is desired. To make steel bars, a bloom is first formed into a billet which is a square typically 2" to 6" in cross-section and approximately 30" long.
The billet is heated to a temperature of approximately 2300.degree.-2400.degree. F. so that it becomes pliable enough to be rolled into a bar shape. The rolling process produces an elliptical shape which is gradually rounded as it passes through subsequent rolling stages. After the billet has been rolled it is much longer and narrower. By way of example, a 5" cross-section billet can be rolled into a round steel bar approximately 5/8" in diameter and hundreds of feet long. The steel bar is then cut into pieces of a desired length for shipment.
Various inspection steps are performed on the billet prior to rolling. These inspection steps in theory detect the existence of flaws in the billet which would produce flaws in the resulting bar product. Once the billet flaws are detected they are removed by scarfing or other procedures. During the rolling of the billet, however, other defects may be introduced. A piece of foreign matter may be stuck to the roll and introduce repetitive elongated scratches or grooves in the steel as rolling occurs. If the rolls become misshaped, they can generate severe irregularities in the steel bar which on subsequent rolling may become folded over to also form elongated grooves or flaws along the bar. If the causes of these defects are not detected as soon as possible, many bars having these defects will be produced.
If the steel bars contain flaws of less than a certain depth, the bars may be sold for their intended purpose. If, however, the flaws exceed a certain depth, the bars are scrapped and must be reprocessed with loss in productivity and at additional expense. It is apparent that it is desirable to detect the presence of flaws as soon as possible so corrective steps can be taken to eliminate the source of the flaws.
Prior patents disclose procedures for detecting the presence of flaws before a product cools. One such U.S. Pat. No. 4,024,470 to Vild et al entitled "Eddy Current Detector for Hot Test Pieces Having Cooling Fluid and Purge Features" discloses apparatus where a combination of a heat shield and a fluid coolant protects detectors from the heat of a hot workpiece.
The Vild et al and a number of other prior patents relating to eddy current testing are assigned to the Republic Steel Corporation, assignee of the present invention. Republic Steel patents which disclose control circuitry and apparatus to classify and mark the position of defects in steel bars are U.S. Pat. Nos. 3,108,230 to Judd et al and 3,263,809 to Mandula et al as well as the '470 patent to Vild et al. An improvement to the marking process of these patents is disclosed in recently issued U.S. Pat. No. 4,365,198 to Toth. U.S. Pat. No. 4,355,281 to Toth et al also concerns an eddy current test arrangement having two detection coils spaced along a workpiece path so that as the workpiece is rotated and translated past the coils flaws are detected. The disclosures of these five patents is incorporated herein by reference.
In a typical eddy current tester, an excitation coil is placed in proximity to a steel object under test. The coil is energized with an electric signal which creates magnetic fields which in turn create eddy currents in the steel. A flaw in the steel disrupts the eddy current flow and this disruption can be sensed by monitoring the induced current in a test coil. A prior art eddy current tester for relatively deep flaws includes both an excitation or energization coil to set up the eddy currents in the workpiece and a separate detector coil which is used in monitoring eddy currents.
Proposals have been made to modify the functioning of these prior eddy current testers. These proposals concern strengthening or enhancing a magnetic field in various locations in relation to a product under test. This enhanced magnetic field produces a stronger response in the test equipment monitoring the eddy currents The prior proposals include either a shield to selectively transmit a magnetic field to the surface of the product or, in the alternative, a rotatable magnetic field producing element moved in relation to the product. U.S. Pat. Nos. 3,152,302 to Allen et al and 4,203,069 to Davis depict these two proposals. These proposals, while recognizing a desirable effect, i.e. the selective enhancement of the response produced by the excitation coil, are believed to be ineffective in producing this result.